1. Field of the Invention
The present invention relates to a novel copolymerized polyester composition. More specifically, the present invention is concerned with a copolymerized polyester composition which does not only excel in heat resistance and rigidity but is also much improved in terms of low-temperature moldability, toughness, alkali resistance and so on, and is suitably used in automotive fields, especially in the form of engine-related parts needed to have heat resistance.
2. Background Information
Heretofore, polyethylene terephthalate has been widely used as materials for fibers, films and various moldings because of their outstanding heat resistance, chemical resistance, mechanical properties, electrical properties and other properties. When such polyethylene terephthalate is used as a molding material, it is well-known to incorporate therein an inorganic filler such as glass fibers for the purpose of bettering the strength, rigidity, heat resistance and like other factors of moldings.
When such glass fiber-reinforced polyethylene terephthalate compositions are to be used for injection molding purposes, however, some problems arise due to their limited rate of crystallization at low temperatures. For instance, when they are injection-molded at a mold temperature of 130.degree. C. or below, they tend to provide moldings poor in surface hardness, rather than well-crystallized moldings. Moreover, when such moldings are used at a temperature exceeding their second-order transition point, they undergo crystallization so excessive that their shape stability deteriorates.
There is also available a certain molding technique which is carried out at a mold temperature of some 50.degree. C. to obtain a molding in which polyethylene terephthalate is not substantially crystallized and which is in turn heat-treated. However, problems with this molding technique are that its productivity drops unavoidably and it provides moldings crystallized by heat treatment and so subjected to volumetric shrinkage, deformation, etc.
Consequently, the polyethylene terephthalate compositions have still been molded using special molding machines with which a mold temperature of 130.degree. C. or higher is usually achieved. Since such molding machines have been unsuitable for general purposes, however, there has been a demand for polyethylene terephthalate compositions so improved in terms of low-temperature moldability that moldings of improved quality can be obtained with a conventional molding machine operable at a mold temperature of 90.degree. C. to up to 110.degree. C.
For that reason, various polyethylene terephthalate compositions having a low-temperature crystallization effect have been proposed until now. For instance, U.S. Pat. No. 4,548,978 teaches a method for incorporating known nucleating agents into polyethylene terephthalate copolymerized with polyalkylene glycols. However, such a method has failed to obtain any moldings of good quality at a mold temperature of 100.degree. C. or lower. In addition, Japanese Patent Publication No. Sho.46-29977 discloses a composition which contains polyethylene terephthalate or glass fiber-containing polyethylene terephthalate with sodium benzoate; Japanese Patent Publication No. Sho.47-14502 discloses a composition which contains lithium terephthalate, lithium stearate or lithium benzoate, and Japanese Patent Publication No. Sho.45-26225 discloses a composition in which an ionic copolymer consisting of an a-olefin and a salt of an .alpha.,.beta.-unsaturated carboxylic acid is added to polyethylene terephthalate. However, such compositions are still less than satisfactory, although improved more or less in terms of low temperature moldability.
Besides having the disadvantage of being poor in low-temperature moldability, polyethylene terephthalate has additional disadvantages of being not only inferior in toughness to polybutylene terephthalate but being also still unsatisfactory in terms of alkali resistance. As copolymerized polyesters in which the toughness of polyethylene terephthalate is improved, a copolymerized polyester obtained by the copolymerization of a polyalkylene glycol with an aliphatic dicarboxylic acid has been proposed (see Japanese Patent Laid-Open Publication No. Sho.62-280221). If glass fibers and a nucleating agent are added to such a copolymerized polyester, then products are obtained, which possess toughness substantially equivalent to that of polybutylene terephthalate. Still, such toughness is not satisfactory altogether. Never until now have any proposals been made of improvements in alkali resistance.
Although excelling in heat resistance and rigidity, the conventional glass fiber-reinforced polyethylene terephthalate compositions are disadvantageous in that they are not only poor in low-temperature moldability and toughness as mentioned above, but are also lacking in alkali resistance.